Shock absorbers dampen or control motion in a vehicle. In conjunction with a vehicle suspension system, shock absorbers control vibrations that occur during driving. If vehicle vibration is unrestrained, vehicle springs continue expanding and contracting until all the energy is absorbed leading to a rough ride. Further, uncontrolled motion from shock creates a great deal of wear on the suspension and steering systems. Shock absorbers minimize this effect.
To absorb unwanted vibrations, shock absorbers are generally disposed between the vehicle body and the suspension system of the vehicle. A piston is located within a pressure tube of the shock absorber, with the piston being secured to the vehicle body by way of a piston rod and the pressure tube being secured to the suspension portion of the vehicle. Because the piston is able, through valving, to limit the flow of damping fluid between opposite sides of the piston, when the shock absorber is compressed or extended, the shock absorber is able to produce a damping force which counteracts the unwanted vibration that would otherwise be transmitted from the vehicle body to the suspension portion.
A shock absorber is a velocity-sensitive hydraulic dampening device. The faster the shock absorber moves, the more resistance it has to the movement. This allows it to automatically adjust to various road conditions. The shock absorber works on a principle of fluid displacement on both its compression and extension cycles. The typical shock absorber has more resistance during its extension cycle then its compression cycle. The extension cycle controls motions of the vehicle body sprung weight. The compression cycle controls the same motions of the unsprung weight.
Typical shock absorbers are valved to offer roughly equal resistance to suspension movement upward (jounce) and downward (rebound). The proportion of a shock absorber's ability to resist these movements is indicated by a numerical ratio that describes what percent of the shock absorber's total control is compression and what percent is extension. For example, typical passenger vehicles use shock absorbers valved at 50% jounce and 50% rebound (50/50). Racing vehicles, on the other hand, use shock valved at about 90% jounce and 10% rebound (90/10). Small vehicles, because of their light-weight and soft springs, require more control in both jounce and rebound in the shock absorbers. Damping rates within the shock absorbers are controlled by the size of the piston, the size of the orifices, and the closing force of the valves.
Conventional hydraulic shock absorbers are available in two styles: a single-tube shock absorber and a dual-tube shock absorber. As illustrated in FIG. 1, the dual-tube shock absorber 10 includes an outer tube called a reserve tube 11 that substantially covers an inner tube called the pressure tube 12. The dual-tube shock absorber 10 includes a fluid reservoir 13 disposed between the pressure tube 12 and the reserve tube 11. A base valve 14 is located between a lower working chamber 15 and the fluid reservoir 13 to limit the flow of fluid between the lower working chamber 15 and the fluid reservoir 13 to produce a damping force, which also counteracts the unwanted vibration that would otherwise be transmitted from the suspension system to the vehicle body. The greater the degree to which the flow of fluid within the shock absorber is restricted by a piston 16 and/or the base valve 14, the greater the damping forces are generated by the shock absorber 10. Thus, a highly restricted flow of fluid would produce a firm ride while a less restricted flow of fluid would produce a soft ride.
In selecting the amount of damping that the shock absorber is to provide, at least three vehicle performance characteristics are considered. These three characteristics are ride comfort, vehicle handling, and road holding ability. Ride comfort is often a function of the spring constant of the main springs of the vehicle as well as the spring constant of the seat, tires and the damping coefficient of the shock absorber. For optimum ride comfort, a relatively low damping force or a soft ride is preferred.
Vehicle handling is related to the variation in the vehicle's attitude including the roll, pitch and yaw. For optimum vehicle handling, relatively large damping forces, or a firm ride, are required to avoid excessively rapid variations in the vehicle's attitude during cornering, acceleration, and deceleration.
Finally, road holding ability is generally a function of the amount of contact between the tires and the ground. To optimize road handling ability, large damping forces, or a firm ride, are required when driving on irregular surfaces to prevent loss of contact between the wheel and the ground for excessive periods of time.
Various types of shock absorbers have been developed with multi-force damping force generating devices to generate the desired damping forces in relation to the various vehicle performance characteristics. Shock absorbers have been developed to provide different damping characteristics depending on the distance or the speed at which the piston moves within the pressure tube. Because of the exponential relation between pressure drop and flow rate, it is a difficult task to obtain a damping force at relatively low piston velocities, particularly at velocities near zero.
The continued development of hydraulic dampers includes the development of multi-force damping force generating devices, which are simpler to manufacture, can be manufactured at a lower cost, and which improve the desired force generating characteristics.
One problem with typical shock absorbers is that they do not damp out smaller, generally smooth road vibrations, which are a source of discomfort to the occupants of the vehicle. Moreover, the typical shock absorbers encourage a higher low velocity dampening force, which impairs vehicle response to smooth road vibrations.
Therefore, it would be desirable to provide a shock absorber that uses the shock absorber internal pressure differential, which is related to a combination of displacement, velocity, and acceleration for valve regulation in controlling dampening for generally smooth road vibrations.